We are accustomed to writing, drawing or painting in different colors on paper, cloth, or other surfaces using crayons, paints, inks, colored chalk, or colored pencils to cite a few examples. Using more modern technology brought on by touch screens and powerful software, we can write on computer screens and make different color images. However, we have not had the privilege of a simple writing pad that imitates paper, devoid of complex electronics and software to hand write or draw images in a multitude of different colors with an untethered stylus such as a pencil or ink pen. Commercial devices such as the Etch A Sketch or the Magna Doodle have not offered multicolor capability.
A considerable improvement on writing pads was made with the discovery of bistable cholesteric liquid crystals (see U.S. Pat. No. 5,453,863) that could be switched by low voltage DC pulses by changing the magnitude of the pulse rather than the frequency. Furthermore, it was discovered that the pressure of a stylus could be used to write a planar line on a focal conic bistable cholesteric layer (see U.S. Pat. No. 6,104,448) that could then be erased with a low voltage DC pulse. There were found to be many other advantages of the bistable materials in that an image created on the writing pad display did not degrade with time and would last indefinitely until erased. The erasure time was found to be less than a second making the bistable cholesteric liquid crystal display a substantially more practical device for a writing pad but with limitations as will be described later.
Cholesteric liquid crystalline materials are unique in their optical and electro-optical features. They can be tailored to Bragg reflect light at a pre-selected wavelength and bandwidth, as these materials possess a helical structure in which the liquid crystal (LC) director twists around a helical axis. The reflected light is circularly polarized with the same handedness as the helical structure of the LC. If the incident light is not polarized, it will be decomposed into two circular polarized components with opposite handedness and one of the components reflected.
The cholesteric material is typically electrically switched to either one of two stable textures; planar or focal conic as described, for example in U.S. Pat. No. 5,453,863. In the planar texture, the director of the LC is uniformly parallel to the plane of the substrates across the cell but has a helical twist perpendicular to the plane of the substrates. It is the helical twist of the uniform planar texture that Bragg reflects light in a selected wavelength band. The focal conic texture contains defects that perturb the orientation of the liquid crystalline helices. In the typical focal conic texture, the defect density is high; thus the helical domain size becomes small and randomized in orientation such that it is just forward scattering and does not reflect impinging light. Once the defect structures are created, they are topologically stable and cannot be removed unless by some external force such as an electric field. Thus, the focal conic texture remains stable and forward scatters light of all wavelengths into an absorbing (usually black) background. These bistable structures can be electronically switched between each other at rapid rates (on the order of milliseconds). Gray scale is also available through various switching schemes in order to adjust the density of reflective helical domains that are oriented perpendicular to the substrates (planar texture) relative to the randomized forward scattering domains (focal conic texture).
Bistable cholesteric liquid crystal displays have several important electronic drive features that other bistable reflective technologies do not. Of extreme importance for addressing a matrix of many pixels in a display is the characteristic of a voltage threshold. A threshold is used for multiplexing a row/column matrix without the need of an expensive active matrix (transistor at each pixel). Bistability with a voltage threshold allows very high-resolution displays to be produced with low-cost passive matrix technology. Gray scale capability allows stacked red, green blue, high-resolution displays with full-color capability where as many as 4096 colors have been demonstrated.
In a cholesteric liquid crystal display, the liquid crystal is typically sandwiched between two substrates that are spaced to a particular gap. The substrates can be either glass or plastic. The bottom substrate is painted with a light absorbing (black or colored) back layer. The cell gap is usually set by plastic or glass spacers that are either cylindrical or spherical in shape. However, when one presses on the top substrate, the liquid crystal is displaced (since fluids are not very compressible) and induced to flow laterally out of the area. Of principle interest is that when the focal conic texture of the liquid crystal is induced to flow, the resulting texture is the planar state. The reflective planar state contrasts well with the dark focal conic background. This is a principle behind U.S. Pat. No. 6,104,448 “Pressure Sensitive Liquid Crystalline Light Modulating Device and Material” which discloses that application of a mechanical stress to the liquid crystalline light modulating material changes an initial light scattering focal conic texture to the light reflecting planar texture. The U.S. Pat. No. 6,104,448 discloses a polymer network that is soluble with the chiral nematic liquid crystal and phase separates to form separated polymer domains. The patent states that the polymer network is distributed in phase separated domains in the cell in an amount that stabilizes the thickness of the structure of the cell. Writing tablets of the prior art are made of a single liquid crystal layer with flexible plastic substrates. Pressure with a stylus draws a monochromatic color image which is the reflective color of the cholesteric planar texture on a black or contrasting color background. The image is erased by applying a voltage pulse to electrodes on the cell that drives the entire cell to the focal conic state.